It is commonly known to use a so-called clay gun to close the taphole of a furnace and an oxygen lance to open it. In the use of a clay gun to plug the taphole of a furnace, clay is introduced under pressure into the taphole at the end of the casting period and such clay hardens to form a plug which stops the flow of molten metal. The clay plug is removed by drilling or by means of an oxygen lance when casting is resumed. More recently, it has been proposed to heat the taphole by means of a small furnace placed around the taphole so as to melt the metal in the taphole and permit it to flow, and to stop heating to block the flow of molten metal at the end of the casting period. The opening of tapholes by drilling or by means of an oxygen lance as in the first above-mentioned method is time consuming while, with the second above-mentioned method, stopping the flow of molten metal by cooling is unreliable.